Ultrasonic Dispersion Observed by Optical Diffraction. II. Ammonia

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Abstract
The velocity of sound in NH3 at 300°K has been measured at a frequency of about 1 MHz over the pressure range from 6 atm down to ½ atm, employing the optical diffraction method. All measured velocities were lower than the low-frequency ideal-gas limit of 436 m sec−1. Interpreting this result as evidence of nonideality and not dispersion, we fitted the data to the formula V2=Vi2(1+sp) and obtained Vi2=(19.122±0.034)×104 m2 sec−2 and s=(−13.9±0.05)×10−3 atm−1. The ideal velocity squared, Vi2, agrees with the spectroscopic value to within 0.05% and shows that no loss of vibrational heat capacity occurs up to f/p=2.1 MHz atm−1. The nonideality correction term s is used to recalculate nonideality corrections to the data of previous workers, and it is suggested that the vibrational relaxation time in NH3 may be of the order of 6 nsec at 300°K.

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